1. Field of the Invention
This invention relates to an imaging optical system having moire eliminating function and more particularly to an imaging optical system suitable for use with a television camera, an electronic still camera, a video endoscope and the like.
2. Description of Related Art
Such optical apparatus as television cameras, electronic still cameras, video endoscopes and the like use image sensors such as solid-state image pickup devices, image pickup tubes and the like to obtain color images. In these optical apparatus, spurious signals called moire or aliasing are produced owing to the beats caused between the sampling frequency determined by the pitches of the arrangement of pixels of the solid-state image pickup device or the arrangement of color elements of a color encoding filter, such as a color striped filter, a color mosaic filter and the like, arranged on the light entering side of the solid-state image pickup device or image pickup tube, on the one hand, and the spatial frequency of an image formed on the light receiving surface of these image pickup means, on the other hand. These spurious signals are a dominant cause for a deterioration in the quality of images. Conventionally, in order to eliminate such spurious signals, optical low-pass filters formed by doublerefracting plates such as quartz plates have been arranged in the imaging optical system for forming an object image on the light receiving surface of the image pickup means (see, e.g., U.S. Pat. No. 4,807,981).
The conventional optical low-pass filter is arranged between the image pickup device and the imaging lens. Therefore, when a zoom lens is used as the imaging lens and the object to be imaged has a spatial frequency spectrum which has a large frequency component at a certain spatial frequency, the elimination of the spurious signals is insufficient or impossible except for a specific magnification, so that a substantial deterioration in the quality of images is caused. Such a phenomenon occurs evidently when a television camera is mounted on the eyepiece portion of a fiberscope. This example will be described in detail.
FIG. 1 shows schematically the eyepiece portion of a fiberscope on which a television camera is mounted. On the eyepiece portion of an endoscope 3 including an image guide fiber bundle 1 and an eyepiece 2 is mounted a television camera having an imaging lens 4, an optical low-pass filter 5 formed by double-refracting plates, and a CCD image pickup device 6. An object image formed on the light emerging end surface of the image guide fiber bundle 1 is imaged again on the light receiving surface of the CCD image pickup device 6 through the eyepiece 2, the imaging lens 4 and the optical low-pass filter 5 to pick up the image. As is well known, the image guide fiber bundle is formed by a large number of optical fibers closely bound together. FIG. 2 shows an enlargement of its light emerging end surface; only the core portions 8 of the regularly arranged fibers appear light. Thus, the image formed on the light emerging end surface is considered as the light pattern of these core portions 8 modulated by the brightness of the object. The spatial frequency spectrum of this object image has a large high frequency component centering around a fundamental frequency determined by the pattern of the core portions. The interference of this high frequency component with the sampling frequency of the CCD image pickup device 6 causes the spurious signals. If the imaging lens 4 is a zoom lens, the fundamental frequency varies with zooming so that the elimination of the spurious signals is insufficient.
FIG. 3 (A) and (B) illustrates this. In FIG. 3, the imaging lens comprises four lens units: a front lens 9, a variator lens 10 for varying the image magnification, a compensator lens 11 for compensating the displacement of the image position owing to the variation of magnification, and a relay lens 12. The imaging lens can have any magnification between a magnification .beta..sub.LOW (FIG. 3 (A)) and a magnification .beta..sub.HIGH (FIG. 3(B)) by moving the variator lens 10 and the compensator lens 11 along the optical axis. When the light and dark pattern 13 formed by the core portions 8 of the image guide fiber bundle and having a repetition pitch P is imaged by this imaging lens, the image on the light receiving surface has an arbitrary size between the smallest repetition pitch P.times..beta..sub.LOW (FIG. 3(A)) and the largest repetition pitch P.times..beta..sub.HIGH (FIG. 3(B)) and the above-mentioned fundamental frequency also varies between 1/(P.times..beta..sub.LOW ) and 1/(P.times..beta..sub.HIGH).
The optical low-pass filter is to reduce the resolution on the higher frequency side than a predetermined spatial frequency to prevent the interference of the spatial frequency component of an object image and the sampling frequency of an image pickup device. As shown in FIG. 4, let MTF (modulation transfer function) and spatial frequency be plotted along the vertical and horizontal axes, respectively, to illustrate the frequency response of the optical low-pass filter. As shown by the solid line in FIG. 4, if the optical low-pass filter is designed in such a manner that MTF is zero at the fundamental frequency 1/(P.times..beta..sub.LOW), that is, has a trap point at the frequency 1/(P.times..beta..sub.LOW) at the time of low magnification imaging, a sufficient elimination of the spurious signals can be effected at the time of low magnification imaging. However, since MTF has a large value at the fundamental frequency 1/(P.times..beta..sub.HIGH) at the time of high magnification imaging, resolution is not reduced sufficiently so that the spurious signals cannot be eliminated. On the other hand, if the optical low-pass filter is designed in such a manner that MTF is zero at the frequency 1/(P.times..mu..sub.HIGH), that is, has a trap point at the frequency 1/(P.times..mu..sub.HIGH) so that the spurious signals can be eliminated at the time of high magnification imaging, resolution is excessively reduced at the time of low magnification imaging to deteriorate the quality of images because MTF is zero at the frequency 1/(P.times..mu..sub.HIGH) whereas it is preferable that MTF have a large value at a frequency under 1/(P.times..mu..sub.LOW).
Thus, an imaging optical system with an optical low-pass filter arranged between a zoom lens and its image plane poses various problems when an object having a large spectral component at a specific spatial frequency is imaged.
Furthermore, apart from the foregoing, if a plurality of fiberscopes are selectively attached to a single television camera to pick up an object image transmitted through their image guide fiber bundles, the spatial frequency spectrum of the object image may vary because each fiberscope may have its own thickness of fibers of the image guide fiber bundle or its own magnification of the eyepiece. In such cases, whether the imaging lens is a zoom lens or not, an optical low-pass filter arranged between the lens and an image pickup device can eliminate the spurious signals for one fiberscope, but may be useless for other fiberscopes so that substantial spurious signals may be produced.